1. Field of the Invention
This invention relates to a heat-resistant member which comprises a metallic substrate and a ceramic coating layer deposited thereon.
2. Description of the Related Art
The thermal barrier coating which consists in coating the surface of a metallic substrate with a varying sort of heat-resistant .cndot. refractory ceramic material is applied, for example, to heat-resistant alloy members which are used in various fields. As means for coating a metallic surface with a ceramic material, the thermal spraying method, baking method, physical vacuum deposition method, chemical vacuum deposition method, surface oxidation method, or the like have been heretofore utilized. Particularly from the viewpoint of productivity on a commercial scale, the thermal spraying method has been generally applied to the coating of a high melting material with a thick film.
Incidentally, the thermal expansion coefficient of a metallic substrate and that of a ceramic material forming a ceramic coating layer are different approximately by one decimal place. At high temperatures or in an environment of serious thermal fluctuation, thermal stress due to the difference in thermal expansion between the metallic substrate and the ceramic coating layer mentioned above arises in the interface therebetween and tends to induce such phenomena as cracking of the ceramic coating layer or separation of the layer from the substrate. Thus, the practice of interposing a metallic bonding layer as a thermal stress relaxing layer between the metallic substrate and the ceramic coating layer is prevailing.
Also for the formation of such a metallic bonding layer as mentioned above, the thermal spraying method is generally adopted. The thermal spraying method is known in two types, the atmospheric plasma spraying method and the low pressure ambient plasma method. The atmospheric plasma spraying method consists in effecting plasma spraying under the atmospheric ambience. The low pressure ambient plasma spraying method resides in effecting plasma spraying under pressure lower than the atmospheric pressure. For the formation of the metallic bonding layer, the atmospheric plasma spraying method and the low pressure ambient plasma spraying method are both utilized.
The metallic bonding layer formed by the atmospheric plasma spraying method, however, contains pores and oxides at a ratio of about several percent and exhibits a great effect in alleviating thermal stress and nevertheless is at a disadvantage in offering poor resistance to high-temperature oxidation or high-temperature corrosion. As the result of this disadvantage, the metallic substrate is deteriorated. In contrast, the metallic bonding layer produced by the low temperature ambient plasma spraying method has a dense texture and contains pores and oxides at a low ratio. While it excels in the ability to resist high temperature oxidation and high temperature corrosion, it is at a disadvantage in exhibiting only a small effect in relaxing thermal stress and betraying vulnerability to thermal fatigue and thermal impacts. As a result, the thermal barrier coating layer tends to sustain cracks and consequently entail separation and suffers from impairment of the properties which are expected of a heat-resistant member.
The conventional heat-resistant member possessing a ceramic coating layer incurs various encounters various problems as described above, depending on the kind of a metallic bonding layer to be interposed as a thermal stress relaxing layer between the metallic substrate and the ceramic coating layer. The metallic bonding layer produced by the atmospheric plasma spraying method, for example, is at a disadvantage in offering poor resistance to high temperature oxidation and high temperature corrosion. Meanwhile, the metallic bonding layer produced by the low pressure ambient plasma thermal spraying method is at a disadvantage in exhibiting only a small effect in relaxing thermal stress and betraying vulnerability to thermal fatigue and thermal impacts.
The development of a heat-resistant member which possesses a metallic bonding layer excelling in resistance to high temperature oxidation and high temperature corrosion and exhibiting stability to tolerate thermal fatigue and thermal impacts has been longed for.